Method and apparatus for temperature compensating a piezoelectric device

ABSTRACT

A control system for temperature compensating a piezoelectric device. The control system includes a temperature compensating circuit that is operable to receive a control signal corresponding to a desired position of the piezoelectric device and compensate the control signal in response to an estimated temperature proximate the piezoelectric device. A piezoelectric device control circuit is operable to receive the temperature compensated control signal and generate a control signal that is adapted to drive the piezoelectric device to the desired position. The temperature proximate the piezoelectric device may be estimated from an estimated ferroelectric polarization of the piezoelectric device or from a temperature sensor.

TECHNICAL FIELD

[0001] The present invention relates generally to piezoelectric devicesand, more particularly, to an apparatus and method for accuratelycontrolling movement of a piezoelectric device under varying operatingtemperatures.

BACKGROUND ART

[0002] Piezoelectric devices alter their shape in response to an appliedelectric field. An electric field applied in the direction ofpolarization effects an expansion of the piezoelectric material in thesame direction, while a voltage applied in the opposite direction ofpolarization will cause a contraction of the material in that samedirection. Piezoelectric bending actuators, such as thermallypre-stressed bending actuators, use the “bending” action of the actuatorto convert electrical energy into mechanical energy.

[0003] Due to the nature of their construction, however, the performanceof these devices is temperature dependent and presents a problem inapplications such as an engine system where the temperature of theactuator may range from 0° C. to 100° C. during operation. In this widetemperature range, the position of the actuator changes as a function ofapplied voltage and temperature so the actuator must be temperaturecompensated to provide a consistent, reliable and predictable movementor displacement of the actuator in response to the input command signal.

[0004] In the past, piezoelectric actuators applied as fuel systemactuators were temperature compensated by mechanical means, such as byhydraulic compensation. These mechanical methods require complex designsthat add significant product cost and decrease the reliability of thevalve control system.

[0005] Thus, there is a need for a piezoelectric actuator thateliminates the need for complex and unreliable mechanical devices toprovide temperature compensation of the actuator. There is also a needfor a piezoelectric actuator that may be accurately and reliably drivento a desired position in a relatively wide temperature range of theactuator.

DISCLOSURE OF THE INVENTION

[0006] While the invention will be described in connection with certainembodiments, it will be understood that the invention is not limited tothese embodiments. On the contrary, the invention includes allalternatives, modifications and equivalents as may be included withinthe spirit and scope of the present invention.

[0007] In accordance with the principles of the present invention, acontrol system for temperature compensating a piezoelectric deviceincludes a temperature compensating circuit that is operable to receivea control signal from a control signal source that corresponds to adesired position of the piezoelectric device. The temperature controlcircuit is operable to generate a temperature corrected or compensatedcontrol signal in response to an estimated temperature proximate thepiezoelectric device. A piezoelectric control circuit is coupled to thetemperature compensating circuit and operable to generate a controlsignal in response to the control signal generated by the temperaturecontrol circuit to drive the piezoelectric device to the desiredposition in response to the estimated temperature of the piezoelectricdevice.

[0008] The above and other objects and advantages of the presentinvention shall be made apparent from the accompanying drawings and thedescription thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate embodiments of theinvention and, together with a general description of the inventiongiven above, and the detailed description of the embodiments givenbelow, serve to explain the principles of the invention.

[0010]FIG. 1 is a block diagram of a control system for temperaturecompensating a piezoelectric device in accordance with the principles ofthe present invention; and

[0011]FIG. 2 is an alternative embodiment of the temperaturecompensating control system shown in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

[0012] With reference to the figures, and to FIG. 1 in particular, acontrol system 10 is shown in accordance with the principles of thepresent invention for temperature compensating a piezoelectric device12, such as a thermally pre-stressed bending actuator, that is coupledto the control system 10 through a pair of leads 14 a, 14 b. As will bedescribed in detail below, control system 10 is operable to receive acontrol signal on line 16 from a control signal source (not shown) thatcorresponds to a desired position of the actuator 12. In response to thecontrol signal on line 16, the control system 10 applies a voltagerelated control signal to the actuator 12 that has been appropriatelycompensated in response to an estimated temperature proximate theoperating environment of the actuator device 12. In this way, thecontrol system 10 accurately controls displacement of actuator 12 inresponse to receipt of the control signal on line 16 from the controlsignal source (not shown) over a range of actuator operatingtemperatures. While not shown, it will be appreciated that the controlsignal source may be any position control system that can control theposition of a piezoelectric device through a control signal.

[0013] Further referring to FIG. 1, control system 10 includes aone-dimensional empirical map or data structure 18 that is operable toreceive the desired actuator position control signal on line 16 from thecontrol signal source (not shown) as an input to the map 18. In responseto receipt of the control signal on line 16, the data structure 18generates as an output on line 20 a desired control signal that iselectrically compatible with a conventional actuator control circuit 22readily known to those skilled in the art. Data structure 18 may be alook-up table stored in RAM or ROM, a software algorithm or a hardwiredcircuit as will be readily appreciated by those skilled in the art thatis operable to generate as an output on line 20 the desired controlsignal having a value or parameter defined by the empirical map 18 inresponse to the desired actuator position control signal on input line16.

[0014] In accordance with the principles of the present invention,control system 10 includes a temperature compensating circuit, indicatedgenerally at 24, that is coupled to the data structure or map 18 and theactuator control circuit 22. The temperature compensating circuit 24 isoperable to receive the control signal on line 20 generated as an outputof the data structure 18, and to generate a temperature compensatedcontrol signal on line 26 that is corrected or compensated in responseto an estimated temperature proximate the operating environment of thepiezoelectric device 12.

[0015] In particular, there is a hysteresis involved in the relationshipbetween the magnitude of the control signal applied to the actuator 12,i.e., the control voltage, and the displacement of the actuator 12 inresponse to that control signal. Since piezoelectric devices are notlinear gain devices, the physical motion profile of the device is notdirectly proportional to the profile of the control signal applied tothe actuator. Moreover, the hysteresis curve is temperature dependent sothat an input control signal applied to an actuator to produce a desireddisplacement at one temperature of the actuator will not produce thesame displacement of the actuator at a different temperature. To thisend, the temperature compensating circuit 24 is operable to correct orcompensate the control signal on line 20 in response to the estimatedtemperature proximate the operating environment of the actuator 12 togenerate a temperature corrected or compensated control signal on line26 that will drive the actuator 12 to the desired position ordisplacement at the estimated temperature proximate the actuator.

[0016] In particular, the temperature compensating circuit 24 of controlsystem 10 includes a two-dimensional empirical map or data structure 28that is operable to receive the desired actuator position control signalon line 20 from the one-dimensional map or data structure 18 as an inputto the map 28. The map 28 is also operable to receive an estimatedtemperature proximate the actuator 12 on line 30 as another input to themap 28. In response to receipt of the desired actuator position controlsignal on line 20 and the estimated temperature proximate the actuator12 on line 30, the data structure 28 generates as an output on line 26the temperature corrected or compensated control signal that will drivethe actuator-12 to the desired position or displacement in response tothe estimated temperature proximate the actuator 12. The temperaturecorrected or compensated control signal on line 26 is coupled to theactuator control circuit 22 to drive the actuator 12 to the desiredposition or displacement at the estimated temperature proximate theactuator 12. Data structure 28 may also be a look-up table stored in RAMor ROM, a software algorithm or a hardwired circuit as will be readilyappreciated by those skilled in the art that is operable to generate asan output on line 26 the temperature corrected or compensated controlsignal on line 26 having a value or parameter defined by the empiricalmap 28 in response to the desired control signal on input line 20 andthe estimated temperature proximate the actuator 12 on input line 30.

[0017] In accordance with one aspect of the present invention, thetemperature proximate the actuator 12 may be estimated from an estimatedferroelectric polarization of the actuator 12. More particularly, thetemperature compensating circuit 24 of control system 10 typicallyincludes a polarization estimating circuit, indicated generally at 32,that is coupled to the data structure or map 28 and the actuator controlcircuit 22. For a given duration of time, the polarization estimatingcircuit 32 measures the change in applied voltage to the actuator 12 (V)through a comparator circuit 34 coupled to the actuator control circuit22 through line 36. During that same duration of time, the polarizationestimating circuit 32 measures the change in charge on the actuator 12(Q) through a current integrating circuit 38 coupled to actuator controlcircuit 22 through line 40. From the measured V and Q values, anequivalent capacitance of the actuator 12 (C_(E)) is determined by thefollowing equation, where the equivalent capacitance (C_(E)) isdependent on the physical construction of the actuator 12 and on thetemperature proximate the operating environment of the actuator 12:

C _(E) =Q/V

[0018] To ensure an accurate estimation of the equivalent capacitance(C_(E)), V must be a significant portion (>50%) of the full travel rangeof the actuator 12. The voltage vs. charge hysteresis curve of theactuator will determine the minimum value of V that can be used in thiscalculation without a loss of accuracy.

[0019] The polarization estimating circuit 32 of control system 10includes a one-dimensional empirical map or data structure 42 that isoperable to receive the equivalent capacitance (C_(E)) on line 44 as aninput to the map 42. The empirical map or data structure 42 ispreferably derived from the ferroelectric polarization hysteresis curvesof the actuator 12 to generate the estimated temperature on line 30. Theequivalent capacitance (C_(E)) effectively gives a unique slope value onthe ferroelectric polarization hysteresis curves that can be correlatedto the estimated temperature proximate the actuator 12 as will beappreciated by those skilled in the art. Data structure 42 may also be alook-up table stored in RAM or ROM, a software algorithm or a hardwiredcircuit as will be readily appreciated by those skilled in the art thatis operable to generate as an output on line 30 the estimatedtemperature having a value or parameter defined by the empirical map 42in response to the equivalent capacitance (C_(E)) on input line 44.

[0020] Alternatively, as shown in FIG. 2 where like numerals representlike parts, a control system 10′ is shown in accordance with anotheraspect of the present invention for temperature compensating theactuator 12. In this embodiment, the polarization estimating circuit 32of FIG. 1 is replaced with a temperature sensor 46 that is adapted to bemounted in the general operating environment of the actuator 12. Forexample, in an engine system, the temperature sensor 46 could be mountedto sense engine oil or coolant temperature, or the sensor could bemounted directly on the actuator 12. In this way, the sensor 46generates an estimated temperature proximate the operating environmentof actuator 12 as an input on line 30 to the two-dimensional empiricalmap or data structure 28. The map 28 is operable to receive the desiredactuator position control signal on line 20 from the one-dimensional mapor data structure 18 as an input to the map 28. The map 28 is alsooperable to receive the estimated temperature proximate the actuator 12on line 30 as another input to the map 28. In response to receipt of thedesired actuator position control signal on line 20 and the estimatedtemperature proximate the actuator 12 on line 30, the data structure 28generates as an output on line 26 the temperature corrected orcompensated control signal that will drive the actuator 12 to thedesired position or displacement in response to the estimatedtemperature proximate the actuator 12. The temperature corrected orcompensated control signal is coupled on line 26 to the actuator controlcircuit 22 to drive the actuator 12 to the desired position ordisplacement at the estimated temperature proximate the actuator.

[0021] Industrial Applicability

[0022] In use, it will be appreciated that control system 10 is operableto provide a temperature corrected or compensated control signal to theactuator 12 to drive the actuator 12 to the desired position ordisplacement in response to the estimated temperature proximate theactuator. The temperature compensating circuit 24 of control system 10eliminates the need for complex and unreliable mechanical devices toprovide temperature compensation of the actuator 12. Additionally, thepolarization estimating circuit 32 of FIG. 1 eliminates the need for atemperature sensor 46 (FIG. 2) to estimate the temperature proximate theoperating environment of the actuator 12. The control system 12 of thepresent invention provides accurate movement control of the actuator 12under varying operating temperatures.

[0023] While the present invention has been illustrated by a descriptionof various embodiments and while these embodiments have been describedin considerable detail, it is not the intention of the applicants torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. The invention in its broader aspects istherefore not limited to the specific details, representative apparatusand method, and illustrative example shown and described. Accordingly,departures may be made from such details without departing from thespirit or scope of applicant's general inventive concept.

[0024] Other aspects, objects and advantages of the present inventioncan be obtained from a study of the drawings, the disclosure and theappended claims.

1. An apparatus for temperature compensating a piezoelectric device,comprising: a temperature compensating circuit operable to receive afirst control signal corresponding to a desired position of thepiezoelectric device and generate a second control signal in response tothe first control signal that is compensated in response to an estimatedtemperature proximate the piezoelectric device; and a piezoelectricdevice control circuit coupled to said temperature compensating circuitand operable to receive the second control signal and generate a thirdcontrol signal in response to the second control signal that is adaptedto drive the piezoelectric device to the desired position.
 2. Theapparatus of claim 1 wherein said temperature compensating circuitincludes a first data structure operable to correlate the first controlsignal with the estimated temperature proximate the piezoelectric deviceto generate the second control signal.
 3. The apparatus of claim 2wherein said temperature compensating circuit further includes apolarization estimating circuit coupled to said piezoelectric devicecontrol circuit and operable to estimate ferroelectric polarization ofthe piezoelectric device.
 4. The apparatus of claim 3 wherein saidtemperature compensating circuit further includes a second datastructure operable to estimate the temperature proximate thepiezoelectric device from the estimated ferroelectric polarization ofthe piezoelectric device.
 5. The apparatus of claim 3 wherein saidpolarization estimating circuit includes a comparator circuit operableto measure a change in voltage applied to the piezoelectric device overa predetermined duration of time.
 6. The apparatus of claim 5 whereinsaid polarization estimating circuit further includes an integratorcircuit operable to integrate current flowing in the piezoelectricdevice over the predetermined duration of time.
 7. The apparatus ofclaim 1 wherein said temperature estimating circuit includes atemperature sensor operable to estimate temperature proximate thepiezoelectric device.
 8. The apparatus of claim 7 wherein saidtemperature compensating circuit includes a first data structureoperable to correlate the first control signal with the estimatedtemperature proximate the piezoelectric device to generate the secondcontrol signal.
 9. An apparatus for temperature compensating apiezoelectric device, comprising: a first data structure operable tocorrelate a first control signal corresponding to a desired position ofthe piezoelectric device with an estimated temperature proximate thepiezoelectric device to generate a second control signal in response tothe first control signal that is compensated in response to theestimated temperature proximate the piezoelectric device; and apiezoelectric device control circuit operable to receive the secondcontrol signal and generate a third control signal in response to thesecond control signal that is adapted to drive the piezoelectric deviceto the desired position.
 10. The apparatus of claim 9 further includinga polarization estimating circuit coupled to said piezoelectric devicecontrol circuit and operable to estimate ferroelectric polarization ofthe piezoelectric device.
 11. The apparatus of claim 10 furtherincluding a second data structure operable to estimate the temperatureproximate the piezoelectric device from the estimated ferroelectricpolarization of the piezoelectric device.
 12. The apparatus of claim 10wherein said polarization estimating circuit includes a comparatorcircuit operable to measure a change in voltage applied to thepiezoelectric device over a predetermined duration of time.
 13. Theapparatus of claim 12 wherein said polarization estimating circuitfurther includes an integrator circuit operable to integrate currentflowing in the piezoelectric device over the predetermined duration oftime.
 14. The apparatus of claim 9 further including a temperaturesensor operable to estimate temperature proximate the piezoelectricdevice.
 15. A method of temperature compensating a piezoelectric device,comprising: receiving a first control signal that corresponds to adesired position of a piezoelectric device; estimating temperatureproximate the piezoelectric device; and generating a control signaladapted to drive the piezoelectric device to the desired position,wherein the control signal is temperature compensated in response to theestimated temperature proximate the piezoelectric device.
 16. The methodof claim 15 wherein the step of generating the control signal comprises:generating a second control signal in response to the first controlsignal that is compensated in response to the estimated temperatureproximate the piezoelectric device; and generating a third controlsignal in response to the second control signal that is adapted to drivethe piezoelectric device to the desired position.
 17. The method ofclaim 15 further comprising: estimating ferroelectric polarization ofthe piezoelectric device; and estimating temperature proximate thepiezoelectric device from the estimated ferroelectric polarization ofthe piezoelectric device.
 18. The method of claim 15 further comprisingestimating temperature proximate the piezoelectric device from atemperature sensor.
 19. The method of claim 16 further comprising:providing a first data structure operable to correlate the first controlsignal with the estimated temperature proximate the piezoelectric deviceto generate the second control signal.
 20. The method of claim 17further comprising: providing a second data structure operable toestimate temperature proximate the piezoelectric device from theestimated ferroelectric polarization of the piezoelectric device.